为什么在 C 中这样定义“max"宏? [英] Why is the 'max' macro defined like this in C?

问题描述

 #define max(a,b) 
   ({ typeof (a) _a = (a); 
       typeof (b) _b = (b); 
     _a > _b ? _a : _b; })

为什么不简单地 (a>b ? a : b)?

推荐答案

因为否则 max(f(1), f(2)) 会调用这两个函数之一两次:

because otherwhise max(f(1), f(2)) would call one of the two functions twice:

f(1) > f(2) ? f(1) : f(2)

而是通过缓存" _a 和 _b 中的两个值

instead by "caching" the two values in _a and _b you have

({
    sometype _a = (a);
    sometype _b = (b);

    _a > _b ? _a : _b;
})

(很明显，正如其他人指出的那样，自动增量/自动减量存在同样的问题)

(and clearly as other have pointed out, there is the same problem with autoincrement/autodecrement)

我认为 Visual Studio 不支持这种方式.这是一个复合语句.阅读此处msvc 是否具有 gcc ({ }) 的模拟

I don't think this is supported by Visual Studio in this way. This is a compound statement. Read here does msvc have analog of gcc's ({ })

我将在此处给出的 gcc 手册中添加复合语句的定义 http://gcc.gnu.org/onlinedocs/gcc-2.95.3/gcc_4.html#SEC62 显示的代码与 max 的问题非常相似:-)

I'll add that the definition of compound statement in the gcc manual given here http://gcc.gnu.org/onlinedocs/gcc-2.95.3/gcc_4.html#SEC62 shows a code VERY similar to the one of the question for max :-)

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